Hi, yes, I've tried with extensions disabled and so on. It's just standard Opera, no turbo or VPN turned on. This is what the help says:

'When you connect to a secure website, the server hosting that site presents your browser with something called a "certificate" to verify its identity. This certificate contains identity information, such as the address of the website, which is verified by a third party that your computer trusts. By checking that the address in the certificate matches the address of the website, it is possible to verify that you are securely communicating with the website you intended, and not a third party (such as an attacker on your network).

You cannot visit davidbowienews.com right now because the website uses HSTS. Network errors and attacks are usually temporary, so this page will probably work later.'

I was able to visit the site fine with Opera (which is my only browser) up until about three weeks ago.

I can easily visit that page so it has to be caused by something local on your end. Speaking of certificates: Opera relies on your local certificate store, and those are being kept up to date by your OS, in this case Windows. So: do you keep your Windows environment up to date?